Search

Microphytobenthos (MPB) are a key primary producer of intertidal mudflats. MPB face strong variability in incident irradiance during low tides. Despite photoprotection and photoacclimation, such variations can translate into photoinhibition of MPB cells. This study explores the effect of photoinhibition on MPB primary production (PP) over a large and productive temperate mudflat (Brouage mudflat, NW France). We used a regional and high-resolution tri-dimensional hydrodynamic model coupled to a MPB model with or without photoinhibition. Photoinhibition leads to a 20% (-0.79 t C) decrease of the simulated MPB PP over the entire mudflat. As the upper shore is exposed to light more frequently and longer than the lower shore, the decrease of MPB PP is higher on the upper shore (-29%) than on the lower shore (-5%). With the highest photosynthetically active radiation cumulated over the mudflat, the decrease of MPB PP due to photoinhibition is the highest during spring and spring tides (-22% and -23%, respectively). The model suggests MPB photoinhibition is sensitive to the photoacclimation status of MPB cells through the light saturation parameter. This first modeling attempt to account for MPB photoinhibition is highly constrained by our current theoretical knowledge and limitations on the MPB growth physiology, but it suggests that this process can have a substantial impact on the MPB PP. As such, assessing the MPB photosynthetic response to the highly variable environmental conditions that prevail in large and productive intertidal mudflats is a real challenge for quantifying MPB PP from a synoptic to inter-annual time scale. Plain Language Summary Benthic micro-algae or microphytobenthos (MPB) inhabiting the surficial sediment sustain the high biological production of intertidal mudflats. MPB achieve photosynthesis by aggregating into a dense biofilm at the mud surface during daytime low tides. As MPB can be exposed to short-term variations and high light levels, they change their short-term physiology and position within the sediment to protect themselves. However, such strategies can be outbalanced by a too long stressful light exposure. In this study, we explore with a numerical model the impact of photoinhibition on MPB primary production (PP) over a large and very productive mudflat (NW France). The model suggests that photoinhibition can strongly impact MPB PP. With photoinhibition, the yearly PP decreases by 20 % over the whole mudflat. The model suggests MPB PP is sensitive to the photoacclimation status of MPB cells, i.e. their light use efficiency at a given light level. This first modeling attempt to account for MPB photoinhibition is highly constrained by our current theoretical knowledge and limitations on the MPB growth physiology, but it suggests that this process can have a substantial impact on the MPB PP.

In recent years diatoms have emerged as a major algal source for the production of bioactive compounds. Marine diatoms grow quickly and can store high amount of lipids. Unfortunately, they are little studied and underexploited resources. The current work deals with an original and rarely investigated source of diatoms: intertidal mudflats. It aims to evaluate the lipid production potential of some strains of benthic diatom species, isolated and hosted in the Nantes Culture Collection (NCC) when cultivated in an airlift photobioreactor. Six strains known for their high biomass and/or lipid productivity: Amphora sp. (NCC169), Entomoneis paludosa (NCC18.2), Nitzschia alexandrina (NCC33), Nitzschia sp. (NCC109), Opephora sp. (NCC366), and Staurosira sp. (NCC182) were cultivated in airlift photobioreactors for the first time. Their lipid class composition, fatty acid, and sterol distribution were studied. Total lipid production varied from 11.4 (Amphora sp.) to 41%DW (Staurosira sp.). Neutral lipid amounts varied from 23 (Amphora sp.) to 76% (Staurosira sp.) of total lipids (%TL). Glycolipids ranged from 18 (Staurosira sp.) to 59%TL (Opephora sp.) and phospholipids accounted for 6 (Staurosira sp.) to 26%TL (Amphora sp.). Some qualitative and quantitative differences were identified in both fatty acid and sterol composition in the different strains analyzed. Staurosira sp. seems to be the most promising species in terms of lipid production and most particularly in triacylglycerol production. Entomoneis paludosa produced phytosterols and eicosapentaenoic acid (EPA), compounds with potential for application in the pharmaceutical sector. Nitzschia alexandrina produced squalene and low levels of saturated fatty acids which could both be interesting in the nutraceutical industry as antioxidants.

Simple Summary Diatoms are microalgae known for their ecological importance. Among them, just a few species are able to produce a blue pigment. We describe Haslea silbo sp. nov., a cosmopolitan species of blue diatoms, found on both sides of the Atlantic Ocean. The description includes the use of both microscopy and next generation sequencing. It has been possible to observe its reproduction in the laboratory, and the blue pigment it produces has also been studied. Abstract Specimens of a new species of blue diatoms from the genus Haslea Simonsen were discovered in geographically distant sampling sites, first in the Canary Archipelago, then North Carolina, Gulf of Naples, the Croatian South Adriatic Sea, and Turkish coast of the Eastern Mediterranean Sea. An exhaustive characterization of these specimens, using a combined morphological and genomic approach led to the conclusion that they belong to a single new to science cosmopolitan species, Haslea silbo sp. nov. A preliminary characterization of its blue pigment shows similarities to marennine produced by Haslea ostrearia, as evidenced by UV–visible spectrophotometry and Raman spectrometry. Life cycle stages including auxosporulation were also observed, providing data on the cardinal points of this species. For the two most geographically distant populations (North Carolina and East Mediterranean), complete mitochondrial and plastid genomes were sequenced. The mitogenomes of both strains share a rare atp6 pseudogene, but the number, nature, and positions of the group II introns inside its cox1 gene differ between the two populations. There are also two pairs of genes fused in single ORFs. The plastid genomes are characterized by large regions of recombination with plasmid DNA, which are in both cases located between the ycf35 and psbA genes, but whose content differs between the strains. The two sequenced strains hosts three plasmids coding for putative serine recombinase protein whose sequences are compared, and four out of six of these plasmids were highly conserved.

DNA barcoding and metabarcoding is increasingly used to effectively and precisely assess and monitor biodiversity in aquatic ecosystems. As these methods rely on data availability and quality of barcode reference libraries, it is important to develop and follow best practices to ensure optimal quality and traceability of the metadata associated with the reference barcodes used for identification. Sufficient metadata, as well as vouchers, corresponding to each reference barcode must be available to ensure reliable barcode library curation and, thereby, provide trustworthy baselines for downstream molecular species identification. This document (1) specifies the data and metadata required to ensure the relevance, the accessibility and traceability of DNA barcodes and (2) specifies the recommendations for DNA harvesting and for the storage of both voucher specimens/samples and barcode data, (undefined)

Thanks are due to FCT/MCTES for the financial support to CESAM (UIDP/50017/2020 + UIDB/50017/2020), through national funds, to JS, and to the NERC Blue-coast project to DMP (NE/N016009/1).

International audience; Estuarine intertidal flats strong biological productivity is mainly based on the activity of benthic microalgae communities or microphytobenthos (MPB), mostly dominated by diatoms. Epipelon is a major MPB growth form comprising motile species, which perform repeated "vertical migration" patterns in the upper sediment layers according to tidal and diurnal cycles with upward migration at the beginning of the daylight emersion and downward migration before immersion starts. Although this fascinating behavior has been extensively studied for more than a century, many of its features remain uncharacterized. Epipelon migratory rhythms are believed to be driven by an endogenous internal clock of unknown nature in combination with diverse environmental stimuli. Among the environmental stimuli impacting on MPB vertical migration, light is probably the most important. Rhythmic changes in surface abundance of natural MPB assemblages were therefore continuously assessed at high frequency by Imaging-PAM fluorimetry in fresh sediment sampled at different seasons, comprising 85 migration profiles from 40 sediment samplings over 2 years, and exposed to different light conditions without any other environmental stimuli (i.e., no tidal-like water flow and stable optimal temperature). In particular, we manipulated (i) the 24-h natural photoperiod MPB that was acclimated to in order to disentangle the tight link between the diurnal and tidal rhythmicity of epipelon migration, and (ii) the light spectrum in order to potentially influence MPB accumulation at the surface of sediment. We found that the migration rhythmicity mapped onto the tidal cycle but that it was modulated, and even overridden, by the diurnal cycle and by the irradiance level during daytime periods with a positive phototactic upward migration up to a certain threshold (in our conditions, 120 µmol photons m −2 s −1 of white light). Also, we found blue wavelengths (465 nm) triggered MPB surface accumulation, as compared to other wavelengths (white, green, and red) in patterns that were intensity-dependent and species-dependent. In particular, we foundtwo species, Navicula spartinetensis and Gyrosigma fasciola, which strongly migrate up under blue light and could potentially be used as model species for further studying the light-responses of intertidal MPB.

The Authie estuary, located at the eastern part of the English Channel is of environmental, ecological, economic and societal importance. With the intention to better understand the sediment dynamic it is important to better assess the role of sediment dynamics including erosion, stabilization and sediment reworking processes which is challenging in such complex environment. It is also important to consider biogenic components such as the microphytobenthos (MPB) distribution, as the primary productivity may play an important role with the bio-stabilization process. As a consequence, there is a crucial need to provide a synoptic overview of inherent bio-physical characteristics of sediments (i.e., composition, water content, grain-size, and biomass) in estuarine environment by generating precise quantitative maps for predicting in a second step estuarine evolution by including sediment transport, sedimentation rates, coastal flows processes and sea level rise caused by climate change for instance. The use of the remote sensing technology is increasingly used for mapping estuarine and coastal environments by providing a synoptic overview of bio-physical characteristics of sediments. In that sense, the combination between remote sensing imaging, topographic data (LiDAR) and in situ measurements is suitable for improving our understanding of sediment dynamics with respect to physical and biological forcings. The main objective of this study is to demonstrate that the synergy between multispectral (i.e., SPOT 6–7 [1.5 m/pixel]; Sentinel-2, 10–60 m/pixel, 5–10 days)”, hyperspectral [Hyspex, 70 cm/pixel, 160 spectral bands] remote sensing images may be suitable for generating both reliable sedimentary and primary productivity budgets; at least for surficial sediments. All presented data were acquired during the same day (09/21/2017) in the framework TeleEST, CPER MARCO and CNRS-OMPBI projects.

The selection of suitable and indigenous microalgae species is a fundamental requirement in developing added-value bioactive compounds recoverable in the food, health, and cosmetics markets. In this work, an integrated screening approach was developed to characterize the lipid rate of 33 diatom species (including 15 species studied for the first time) belonging to 16 genera from the Nantes Culture Collection, with the main objective of discovering bioactive lipid producers. For that purpose, a simple reliable method for establishing growth kinetics of strains and semi-quantitative analysis of lipid rates was developed. Growth kinetics measurements were achieved by daily minimal measurement fluorescence (F0) whereas lipid rate analyses were performed by high-throughput Fourier Transform Infrared spectroscopy on entire cells and lipid extracts. Results indicated that the method could be used directly on entire cells in spite of the presence of silica for the FTIR approach (due to frustule). The total lipid rate was species-dependant and ranged from 3.7% to 30.5% DW. Six strains out of 33 were found to present a higher total lipid rate superior to 15% DW, and 11 showed medium lipid rates ranging from 10% to 15% DW. The results revealed that five diatom species i.e. Amphora sp. NCC169, Nitzschia sp. NCC109, Nitzschia alexandrina NCC33, Opephora sp. NCC366 and Staurosira sp. NCC182 presented interesting growth capabilities and should be further investigated as potential sources for their original lipid rate.

This work is an extension of the MicroPhytoBenthos Optical Model (MPBOM) workflow. The model was based on the observation that the biofilm itself has a negligible inherent reflectance and can be described solely by the ratio between its apparent reflectance (RA) and background reflectance (RB), allowing a straightforward calculation of the absorption coefficient (α). This coefficient is directly related to pigment concentrations estimated by High Performance Liquid Chromatography (HPLC). To run the model, assess and extend the use of α, the background contribution is a critical step. This work shows that: (i) indices based on reflectance and absorption coefficient spectra derived from the optical model correctly identified the main microphytobenthos (MPB) groups covering a pixel; (ii) contrary to the RA index each new α index was insensitive to biomass variations; (iii) for each MPB group there was a significant linear relation between the biomass estimated by HPLC and α peak at 673 nm; (iv) indices based on α spectra were almost insensitive to mixing constraints at a subpixel level. Knowing the background reflectance contribution of MPB biofilms, α can therefore be used to map MPB algal composition and biomass at any scale from MPB synthetized in laboratory to intertidal mudflat airborne observations.

Monitoring photosynthesis is a great challenge to improve our knowledge of plant productivity at the ecosystem level, which may be achieved using remote-sensing techniques with synoptic abilities. The main objective of the current study is to take up this challenge for microphytobenthos (MPB) primary production in intertidal mudflats. This was achieved by coupling hyperspectral radiometry (reflectance,ρ and second derivative, δδ) and PAM-fluorometry (non-sequential light curve, NSLC) measurements. The later allowed the estimation of the primary production via the light use efficiency (LUE) and the electron transport rate (ETR) whereasρ allowed to estimate pigment composition and optical absorption cross-section (a*). Five MPB species representative of the main growth forms: epipelic (benthic motile), epipsammic (benthic motile and non motile) and thycoplanktonic (temporarily resuspended in the water column) were lighted at increasing light intensity from dark to 1950 µmol photons.m-2.s-1. After spectral measurements, a* was retrieved using a radiative transfer model and several radiometric indices were tested for their capacity to predict LUE and ETR. The spectral estimation of these two photosynthetic variables was subsequently compared to the values estimated by PAM-fluorometry. Results showed that different responses related to the xanthophyll cycle (de-epoxydation state) were observed for the three growth-forms with increasing light levels. However, a single relationship with radiometric index was not affected by species/growth-forms, i.e. δδ496/508, called the MPBLUE index to predict LUE and ETR. This index has the potential to be applied to air borne hyperspectral imagery for large-scale assessment of MPB production.

Satellite remote sensing (RS) is routinely used for the large-scale monitoring of microphytobenthos (MPB) biomass in intertidal mudflats, and has greatly improved our knowledge of MPB spatio-temporal variability and its potential drivers. Processes operating at smaller scales however, such as the impact of benthic macrofauna on MPB development, to date remain underinvestigated. In this study, we analysed the influence of wild Crassostrea gigas oyster reefs on MPB biofilm development using multispectral RS. A 30-year time series (1985–2015) combining high resolution (30 m) Landsat and SPOT data was built in order to explore the relationship between C. gigas reefs and MPB spatial distribution and seasonal dynamics, using the normalized difference vegetation index (NDVI). Emphasis was placed on the analysis of a before after control impact (BACI) experiment designed to assess the effect of oyster killing on the surrounding MPB biofilms. Our RS data reveal that the presence of oyster reefs positively affects MPB biofilm development. Analysis of the historical time series first showed the presence of persistent, highly concentrated MPB patches around oyster reefs. This observation was then confirmed by the BACI experiment which showed that killing the oysters (while leaving the physical reef structure, i.e. oyster shells, intact) negatively affected both MPB biofilm biomass and spatial stability around the reef. As such, our results are consistent with the hypothesis of nutrient input as an explanation for the MPB growth promoting effect of oysters, whereby organic and inorganic matter released through oyster excretion and biodeposition stimulates MPB biomass accumulation. MPB also showed marked seasonal variations in biomass and patch shape, size and degree of aggregation around the oyster reefs. Seasonal variations in biomass, with higher NDVI during spring and fall, were consistent with those observed at broader scales in other European mudflats. Our study provides the first multi-sensor RS satellite evidence of the promoting and structuring effect of oyster reefs on MPB biofilms.

Although estuarine microphytobenthos (MPB) is frequently exposed to excessive light and temperature conditions, little is known on their interactive effects on MPB primary productivity. Laboratory and in situ experiments were combined to investigate the shortterm joint effects of high light (HL) and high temperature (37°C versus 27°C) on the operating efficiency of photoprotective processes [vertical migration versus non-photochemical quenching (NPQ)] exhibited by natural benthic diatom communities from two intertidal flats in France (FR) and Portugal (PT). A clear latitudinal pattern was observed, with PT biofilms being more resistant to HL stress, regardless the effect of temperature, and displaying a lower relative contribution of vertical migration to photoprotection and a stronger NPQ in situ. However, higher temperature leads to comparable effects, with photoinhibition increasing to about three times (i.e. from 3% to 10% and from 8% to 22% in PT and FR sites respectively). By using a number of methodological novelties in MPB research (lipid peroxidation quantification, Lhcx proteins immunodetection), this study brings a physiological basis to the previously reported depression of MPB photosynthetic productivity in summer. They emphasize the joint role of temperature and light in limiting, at least transiently (i.e. during emersion), MPB photosynthetic activity in situ.

International audience; In intertidal marine sediments, characterized by rapidly fluctuating and often extreme lightconditions, primary production is frequently dominated by diatoms. We performed a comparativeanalysis of photophysiological traits in 15 marine benthic diatom species belonging to the fourmajor morphological growth forms (epipelon (EPL), motile epipsammon (EPM-M) and non-motileepipsammon (EPM-NM) and tychoplankton (TYCHO)) found in these sediments. Our analysesrevealed a clear relationship between growth form and photoprotective capacity, and identified fastregulatory physiological photoprotective traits (that is, non-photochemical quenching (NPQ) and thexanthophyll cycle (XC)) as key traits defining the functional light response of these diatoms. EPMNMand motile EPL showed the highest and lowest NPQ, respectively, with EPM-M showingintermediate values. Like EPL, TYCHO had low NPQ, irrespective of whether they were grown inbenthic or planktonic conditions, reflecting an adaptation to a low light environment. Our resultsthus provide the first experimental evidence for the existence of a trade-off between behavioural(motility) and physiological photoprotective mechanisms (NPQ and the XC) in the four majorintertidal benthic diatoms growth forms using unialgal cultures. Remarkably, although motilityis restricted to the raphid pennate diatom clade, raphid pennate species, which have adopted anon-motile epipsammic or a tychoplanktonic life style, display the physiological photoprotectiveresponse typical of these growth forms. This observation underscores the importance of growthform and not phylogenetic relatedness as the prime determinant shaping the physiologicalphotoprotective capacity of benthic diatoms.

Microphytobenthic (MPB) biofilms are taxonomically mixed microbial assemblages (e.g. cyanobacteria, chlorophyta and diatoms) responsible for important estuarine ecosystem functions (e.g. nutrient fluxes, sediment cohesion, and primary productivity). MPB biofilms exhibit high spatial–temporal variation leading to high variability in collected data and requiring high sampling replication. Thus there is an increasing interest in developing remote sensing tools to quantify and describe MPB biofilms. The objectives of the current work were to investigate the effect of MPB taxonomic diversity on biofilm spectral reflectance signatures and to investigate the possibility of using second derivative analysis to estimate MPB chlorophyll a (as a proxy for biomass) and pigment composition in monospecific and mixed species biofilms. Two diatoms, Amphora coffeaeformis (Agardh) Kutzing and Cylindrotheca closterium (Ehrenberg) W. Smith; one green algae, Dunaliella tertiolecta and one cyanobacteria, Spirulina platensis (Gomont) Geitler were used to construct artificial MPB biofilms on glass fiber filters. Biofilms with a range of different MPB concentrations and different taxonomic groups were constructed, their spectral reflectances were measured with a ASD FieldSpec 3FR spectroradiometer and pigments were extracted and measured by HPLC. The best relationships between pigment concentration and second derivative spectra (δδ) obtained with monospecific biofilms were: δδ 548 for fucoxanthin, δδ 590 for zeaxanthin, δδ 650 for chlorophyll b and δδ 675 for chlorophyll a . The relationship between δδ 590 and zeaxanthin was probably the result of covariation with phycocyanin or phycoeritrin. δδ 590 and δδ 650 were strongly affected by the presence of other taxonomic groups in the mixed species biofilms and underestimated the respective pigments, while δδ 548 and δδ 675 could be used to estimate directly fucoxanthin and chlorophyll a in taxonomically mixed biofilms. The relationship between δδ 675 and chlorophyll a was strongly affected by the presence of a second derivative double peak caused by chlorophyll fluorescence in the red region and was highly dependent on the smoothing amount applied to the original reflectance spectra. An alternative was proposed using the area of the second derivative peaks between 660 and 690 nm (A[δδ RED ]). This index showed very good relationships with MPB chlorophyll a concentration and was almost insensitive to the amount of smoothing applied to the original spectra.

Estuarine intertidal sediments are colonized by photosynthetic microorganisms grouped under the generic term of microphytobenthos (MPB). These microbial assemblages form transient biofilms at the sediment surface and have important ecosystem functions. MPB biofilms are well known to exhibit high microscale patchiness whereas meso- and macroscale spatio-temporal structures are little known. In this work, satellite remote sensing was used to map MPB assemblages at such scales. MPB interannual distribution was investigated in the poly- and mesohaline domain of the north-European estuary (Loire estuary), using a multispectral SPOT image time series (1991–2009). The Normalized Difference Vegetation Index (NDVI) was calculated from two SPOT channels, XS2 and XS3, (red and near-infrared wavelengths, respectively). MPB biofilms were identified by NDVI values between 0 and 0.3. At the scale of the whole intertidal area, the results showed that MPB biofilms in the Loire estuary exhibited perennial structures in both the polyhaline and mesohaline sectors, occupying nearly 90% of the mudflat surfaces. MPB biofilm density was closely associated with intertidal position, with thicker biofilms developing mostly in the upper and middle shore, and formed kilometric longitudinal structures parallel to the shoreline. Mean NDVI values showed a strong positive correlation with mean seasonal air temperature ( τ = 0.714, p τ = 0.810, p τ = −0.810, p τ = −0.910 in the mesohaline).

International audience; The influence of phenological variations on the reflectance of the intertidal marine angiosperm Zostera noltii was studied using spectroradiometry and pigment analysis. Leaves were sampled each month from March to November and spectral reflectance was measured in the 400-900 nm wavelength range in the laboratory for increasing biomass. High Performance Liquid Chromatography (HPLC) was used to assess the pigment composition and concentration of the leaves and their epiphytes. Several vegetation indices (VIs) were tested for their sensitivity to seasonal variations, from NDVI-like ratios to indices based on derivative analysis. Pigment concentration showed a seasonal effect, with the highest concentrations observed in summer, synchronous with the above-ground biomass peak. The seasonal variations were clearly visible in the leaf reflectance spectrum, showing for the same amount of vegetation, lower reflectance in summer for the visible wavelengths and a higher near-infrared (NIR) plateau compared to the other months. Indices based on the difference between the red and NIR reflectance values, typically all the NDVI-like ratios, were the most sensitive to seasonal variations in pigment concentration. In fact, seasonal effects were systematically reduced for VIs including a blue band correction, namely the modified Specific Ratio (mSR(705)), the modified Normalized Difference (mND(705)), the modified NDVIs for Landsat, FORMOSAT and narrow band NDVI (mNDVI(673)) as well as the Atmospherically Resistant Vegetation Index (ARVI). However, these indices showed a faster saturation for increasing seagrass biomass, except for the mSR(705), which was the least sensitive to biomass saturation. The interesting properties of this index suggest that it should now be tested for airborne or satellite remote sensing mapping of Z. noltii beds but it requires a high spectral resolution and cannot be applied to multispectral satellite images.

A new fast mapping approach based on an optical model was developed and applied to hyperspectral airborne HySpex images of Bourgneuf Bay (French Atlantic coast). The aims were (1) to discriminate diatom biofilms (from the Bacillariophyceae class) constituting microphytobenthos on any mudflat and (2) to estimate their biomass expressed in mg Chl a·m− 2 by remote sensing of poorly accessible mudflats. Due to the distinct absorption peaks of certain pigments of different microalgae classes constituting microphytobenthos, the high spectral resolution of hyperspectral images was employed in order to detect the pigment assemblage characterizing diatom biofilms. Thus, the HySpex 1600 VNIR camera was used both in the laboratory for estimating biomass under controlled conditions and in an airborne setting. 1) The mapping approach used known spectral indices (NDVI and MPBI) and new ones (IDiatom and IEuglenid) to characterize quickly the key pigments of two main microphytobenthos classes, for a fast selection of diatom and euglenid biofilms by two different masks. 2) The inherent spectral properties of biofilms in the 700–900 nm range were also used to calculate an original estimate of the substrate/environmental effects in each mask. 3) Biomass quantification of the diatom biofilms was finally performed by application of the microphytobenthos optical model (MPBOM). This approach was first tested on laboratory HySpex images of synthetic biofilms for which the in vitro biomass measurements by chromatography (HPLC) were available. The comparison between estimated and measured biomass values showed the reliability of the mapping approach for biomass estimation. Finally, a map of diatom biomass was obtained at the scale of the entire Bourgneuf Bay. The range of biomass variation (0–35 mg Chl a·m− 2) was in close agreement with the biomass maps presented in previous studies.

Benthic diatoms are dominant primary producers in intertidal flats. This study investigated the effect of different nitrogen sources and concentrations on Entomoneis paludosa growth and physiological responses. Six nitrogen sources either inorganic (nitrate and ammonium) or organic (urea, arginine, glutamine and glycine) supplied at two concentrations (40 and 400 µM-N) induced significant effects on growth, carbon, nitrogen, pigment content and maximum PSII quantum efficiency (F v /F m). Entomoneis paludosa grew under all nitrogen sources albeit showing differences in lag phase, growth rate and cell yield. Inorganic nitrogen, urea and arginine induced higher growth, whereas glycine did not support high biomass. F v /F m showed variability dependent on nitrogen source and C/N ratio. F v /F m varied between 0.55 and 0.65 at 400 μM-N with the highest values observed in glycine, glutamine and urea, whereas nitrate, ammonium and arginine induced lower F v /F m. All cellular components decreased in the 40 µM-N treatments, with nitrogen and pigments being lower than carbon content. Light-harvesting pigment ratios Chl c/Chl a and photoprotective pigment ratios (diatoxanthin + diadinoxanthin)/Chl a increased, while fucoxanthin/Chl a ratios were unaffected by N-limitations. Entomoneis paludosa was capable of quickly adapting and using a wide variety of nitrogen sources. This adaptability may contribute to microphytobenthos diatom ecological success in mudflat ecosystems.

Remote sensing of terrestrial vegetation uses a wide range of vegetation indices (VIs) to monitor plant characteristics, but these indices can be very sensitive to canopy background reflectance. This study investigated background influences on VIs applied to intertidal microphytobenthos, using a synthetic spectral library constituted by a spectral combination of three contrasting types of sediment (sand, fine sand, and mud) and reflectance spectra of benthic diatom monospecific cultures obtained in controlled conditions. The spectral database exhibited, for the same biomass range (3–182 mg chlorophyll a m − 2 ), marked differences in albedo and spectral contrast linked to sediment variability in water content, grain size, and organic matter content. Several VIs were evaluated, from ratios using visible and near infrared wavelengths, to hyperspectral indices (derivative analysis, continuum removal). Among the ratios, the Normalized Difference Vegetation Index (NDVI) appeared less sensitive to background effects than VIs with soil corrections such as the Perpendicular Vegetation Index (PVI), the Soil-Adjusted Vegetation Index (SAVI), the Modified second Soil-Adjusted Vegetation Index (MSAVI2) or the Transformed Soil-Adjusted Vegetation Index (TSAVI). The lower efficacy of soil-corrected VIs may be explained by the structural differences and optical behavior of soil vs. canopies compared to sediment vs. microphytobenthos biofilms. The background effects were minimized using Modified Gaussian Model indices at 632 nm and 675 nm, and the second derivative at 632 nm, while poor results were obtained with the red-edge inflection point (REIP) and the second derivative at 675 nm. The least sensitive index was the Phytobenthos Index which is very similar to the NDVI, but uses a red wavelength at 632 nm instead of 675 nm, to account for the absorption by chlorophyll c . The modified NDVI 705 , where the 705 nm wavelength replaces the red band, showed moderate background sensitivity. Moreover, the NDVI 705 and the Phytobenthos Index have the additional relevant property of being less sensitive to the index saturation response with increasing biomass. Unfortunately, these VIs cannot be applied to broad-band multispectral satellite images, and require sensors with a hyperspectral resolution. Nevertheless, this study showed that the background influence was not a limitation to applying the ubiquitous NDVI to map intertidal microphytobenthos using multispectral satellite images.

Predictive modelling to map subtidal communities is an alternative to "traditional" methods, such as direct sampling, remote sensing and acoustic survey, which are neither time- nor cost-effective for vast expanses. The principle of this modelling is the use of a combination of environmental key parameters to produce rules to understand species distribution and hence generate predictive maps. This study focuses on subtidal kelp forests (KF) on the coast of Brittany, France. The most significant key parameters to predict KF frequency are (1) the nature of the substrate, (2) depth, (3) water transparency, (4) water surface temperature and (5) hydrodynamics associated with the flexibility of algae in a flow. All these parameters are integrated in a spatial model, built using a Geographical Information System. This model results in a KF frequency map, where sites with optimum key parameters show a deeper limit of disappearance. After validation, the model is used in the context of Climate Change to estimate the effect of environmental variation on this depth limit of KF. Thus, the effects of both an increase in water temperature and a decrease in its transparency could lead to the complete disappearance of KF.

Spatiotemporal changes in microphytobenthos species composition were investigated in relation to structural variables—biomass; life-forms; detritus ratio, pheopigment a (pheo a):chl a; and sediment characteristics—at mesoscale in a shellfish macrotidal ecosystem. To characterize algae assemblages, multivariate analyses were performed (multidimensional scaling [MDS] ordination and clustering), supported by correlations between structural variables. Microphytobenthos, dominated by 97% diatoms belonging to 89 taxa, was constituted by two main assemblages: The first one, composed of common species (occurrence >50%), mainly nanobenthic (size 70 mg chl a · m−2) and low detritus ratio. Conversely, the second assemblage comprised occasional species (occurrence from 10% to 50%), mainly microbenthic (>30 μm) and epipelic (moving cells), occurring during summer at muddy sites. This assemblage, characterized by low diversity, high biomass variability, and high detritus ratio, was dominated by Scolioneis tumida or Plagiotropis vitrea and Navicula spartinetensis. Whereas hydrodynamics globally explained the gradual assemblage changes throughout the entire mudflat and the year, oyster beds and ridge and runnel features appeared to be local spatial structuring factors allowing the establishment of specific assemblages. This study suggests a significant role for epipsammon biomass, until now underestimated, in the functioning of this turbid coastal ecosystem. Table 2. Microphytobenthic taxa identified from samples from the first 2 mm of sediment from Bourgneuf Bay (France) mudflat between May 2000 and July 2001. Taxa Abbreviation Ecologya Size Occurrence (%) aInd, ecology was undetermined. For definition of epipsammic, epipelic, epiphytic, nanobenthic, and microbenthic, see text (Materials and Methods). Common species Plagiogrammopsis vanheurkii (Grunow) Hasle, Stosch et Syvertsen Plagp vanh Epipsammic Nanobenthic 82.50 Amphora spp. Amph spp. Epipsammic Nanobenthic 81.25 Achnanthes hauckiana Grunow Achn hauc Epipsammic Nanobenthic 78.75 Navicula perminuta Grunow Navi perm Epipelic Nanobenthic 76.25 Thalassiosira decipiens (Grunow) Jorg. Thal deci Epipsammic Nanobenthic 76.25 Fallacia tenera (Hust.) D. G. Mann Fall tene Epipelic Nanobenthic 73.75 Achnanthes lanceolata var. elleptica Cleve Achn lanc Epipsammic Nanobenthic 71.25 Cymatosira belgica Grunow Cyma belg Epipsammic Nanobenthic 68.75 Navicula spartinetensis M. J. Sullivan et Reimer Navi spar Epipelic Nanobenthic 68.75 Plagiogramma tenuissimum Hust. Plagg tenu Epipsammic Nanobenthic 66.25 Cocconeis hoffmanni Simonsen Cocc hoff Epipsammic Nanobenthic 65.00 Biremis ambigua D. G. Mann Bire ambi Epipsammic Nanobenthic 58.75 Odontella aurita (Lyngb.) C. Agardh Odon auri Epiphytic Microbenthic 57.50 Navicula anglica Ralfs Navi angl Epipelic Nanobenthic 56.25 Cocconeis scutellum Ehrenb. Cocc scut Epiphytic Microbenthic 55.00 Fragilaria pinnata Ehrenb. Frag pinn Epipsammic Nanobenthic 53.75 Cocconeis pelta A. W. F. Schmidt Cocc pelt Epipsammic Nanobenthic 51.25 Occasional Raphoneis amphiceros Ehrenb. Raph amph Epipsammic Microbenthic 41.25 Actinoptychus undulatus (J. W. Bailey) Ralfs Acti undu Epipsammic Nanobenthic 38.75 Odontella rhombus (Ehrenb.) Kutz. Odon rhom Benthic Microbenthic 37.50 Navicula rostellata Kutz. Navi rost Epipelic Microbenthic 37.50 Brockmaniella brockmanii (Grunow) Hasle, Stosch et Syvertsen Broc broc Epipsammic Nanobenthic 36.25 Plagiotropis vitrea (W. Smith) Cleve Plagt vitr Epipelic Microbenthic 35.00 Paralia marina (W. Smith) Heib. Para marina Epipsammic Nanobenthic 35.00 Delphineis minutissima (Hust.) Simonsen Delp minu Epipsammic Nanobenthic 32.50 Pleurosigma aestuari (Breb.) W. Smith Pleu aest Epipelic Microbenthic 31.25 Dimerogramma minor (W. Greg.) Ralfs Dime minor Epipsammic Nanobenthic 30.00 Podosira stelliger (J. W. Bailey) D. G. Mann Podo stel Epipsammic Microbenthic 30.00 Staurophora amphioxys (W. Greg.) D. G. Mann Stau amph Epipelic Microbenthic 23.75 Nitzschia cf. frustulum (Kutz.) Grunow Nitz frus Epiphytic Nanobenthic 23.75 Gyrosigma fasciola (Ehrenb.) J. W. Griff. et Henfr. Gyro fasc Epipelic Microbenthic 22.50 Scolioneis tumida (Breb.) D. G. Mann Scol tumi Epipelic Microbenthic 22.50 Gyrosigma distortum (W. Smith) Cleve Gyro dist Epipelic Microbenthic 20.00 Craticula cuspidata (Kutz.) D. G. Mann Crat cusp Epipelic Nanobenthic 20.00 Petroneis marina (Ralfs) D. G. Mann Petr mari Epipelic Microbenthic 17.50 Actinoptychus splendens (Shabolt) Ralfs ex A. Pritch. Acti sple Epipsammic Microbenthic 17.50 Navicula phyllepta Kutz. Navi phyl Epipelic Nanobenthic 16.25 Navicula gregaria Donkin Navi greg Epipelic Nanobenthic 15.00 Cocconeis placentula Ehrenb. Cocc plac Epipsammic Nanobenthic 13.75 Euglenophyceae Eugl Benthic Microbenthic 13.75 Pleurosigma angulatum (Quekett) W. Smith Pleu angu Epipelic Microbenthic 12.50 Gyrosigma wansbeckii (Donkin) Cleve Gyro wans Epipelic Microbenthic 12.50 Nitzschia sigma (Kutz.) W. Smith Nitz sigma Epipelic Microbenthic 12.50 Nitzschia cf. panduriformis W. Greg. Nitz pand Epipelic Nanobenthic 11.25 Gyrosigma limosum Sterrenburg et J. C. Underw. Gyro limo Epipelic Microbenthic 10.00 Auxospore (Melosira spp.) Auxo Ind Nanobenthic 10.00 Opephora pacifica (Grunow) P. Petit Opep paci Epipsammic Nanobenthic 10.00 Cerataulus smithi Ralfs ex A. Pritch. Cera smit Epipsammic Microbenthic 10.00 Rare Biddulphia biddulphiana (J. E. Smith) C. S. Boyer Bidd bidd Epipsammic Microbenthic 8.75 Plagiogrammopsis leve (W. Greg.) Ralfs Plagp leve Epipsammic Nanobenthic 8.75 Fragilaria spp. Frag spp. Epipsammic Nanobenthic 8.75 Cyclotella sp. Cycl sp. Ind Microbenthic 8.75 Nitzschia acicularis (Kutz.) W. Smith Nitz acic Epipelic Microbenthic 7.50 Eunotogramma dubium Hust. Euno dubi Epipsammic Nanobenthic 7.50 Gyrosigma scalproides (Rabenh.) Cleve Gyro scal Epipelic Microbenthic 6.25 Plagiotropis elegans (W. Smith) Cleve Plagt eleg Epipelic Microbenthic 6.25 Surirella sp. Suri sp. Epipelic Microbenthic 6.25 Melosira sp. 2 Melo sp. Ind Nanobenthic 6.25 Thalassiosira punctigera (Castracane) Hasle Thal punc Epipsammic Microbenthic 6.25 Gyrosigma balticum (Ehrenb.) Cleve Gyro balt Epipelic Microbenthic 5.00 Melosira cf. jurgensii C. Agardh Melo jurg Epiphytic Nanobenthic 5.00 Auliscus sculptus (Smith) Ralfs ex A. Pritch. Auli scul Epipsammic Microbenthic 5.00 Odontella regia (M. Schultze) Simonsen Odon regia Planktonic Microbenthic 3.75 Entomoneis paludosa (W. Smith) Reimer Ento palu Epipelic Microbenthic 3.75 Navicula ramosissima (C. Agardh) Cleve Navi ramo Epipelic Microbenthic 3.75 Nitzschia closterium (Ehrenb.) W. Smith Nitz clos Planktonic Microbenthic 3.75 Very rare Cylindrotheca gracilis Breb. Cyli grac Benthic Microbenthic 2.50 Navicula cincta Ehrenb. Navi cinc Epipelic Nanobenthic 2.50 Navicula cancellata Donkin Navi canc Epipelic Microbenthic 2.50 Navicula palpebralis Breb. Navi palp Epipelic Microbenthic 2.50 Diploneis sp. 1 Dipl sp. 1 Benthic Nanobenthic 2.50 Diploneis didyma Ehrenb. Dipl didy Epipelic Microbenthic 2.50 Nitzschia sp. Nitz sp. Benthic Microbenthic 2.50 Nitzschia palea (Kutz.) W. Smith Nitz palea Epipelic Microbenthic 2.50 Nitzschia bilobata W. Smith Nitz bilo Epipelic Microbenthic 2.50 Nitzschia constricta (Grunow) Hust. Nitz cons Epipelic Microbenthic 2.50 Nitzschia fasciculata Grunow Nitz fasc Planktonic Microbenthic 2.50 Triceratium antediluvianum (Ehrenb.) Grunow Tric ante Epipsammic Microbenthic 2.50 Triceratium favus Ehrenb. Tric favus Epipsammic Microbenthic 2.50 Grammatophora serpentina (Ralfs) Ehrenb. Gram serp Planktonic Microbenthic 2.50 Campylodiscus echeneis Ehrenb. Camp eche Epipelic Microbenthic 2.50 Cerataulus turgidus Ehrenb. Cera turg Epipsammic Microbenthic 2.50 Chlorophyceae Chlo Benthic Microbenthic 2.50 Scoliotropis latestriata (Breb.) Cleve Scol late Epipelic Microbenthic 1.25 Plagiogramma staurophorum (W. Greg.) Heib. Plagg stau Epipsammic Nanobenthic 1.25 Nitzschia spathulata Breb. Nitz spat Epipelic Microbenthic 1.25 Lithodesmium undulatum Ehrenb. Lith undu Planktonic Microbenthic 1.25 Melosira nummuloides C. Agardh Melo numm Epiphytic Nanobenthic 1.25 Oscillatoriaceae Osci Ind Nanobenthic 1.25

Intertidal mudflats are important feeding areas for migrating and wintering shorebird species. Partial release of organic and inorganic nutrients through bird excretion may enrich the nutrients flow of mudflat ecosystems, especially in areas with a low flushing rate. The objective of the present work was to experimentally evaluate the effect of Dunlin droppings on the growth and biochemical composition of the microphytobenthic diatom, Entomoneis paludosa (W. Smith) Reimer. Different nutritional conditions were tested to investigate the impact on E. paludosa growth rate, biomass, elemental (CHN) and biochemical composition (lipids, proteins, carbohydrates, pigments). Various culture media were used going from plain artificial seawater to F/2 culture media containing variable nitrate concentrations (0; 50 or 882 μM-NO3) to which bird dropping extract was added or not. The faeces extracts contained inorganic nitrogen (9.1 μM-NH4), inorganic phosphorus (8.2 μM-PO4), traces of silicate (0.2 μM-Si), organic nitrogen in the form of urea (16 μM-N) and other dissolved organic nitrogen (120 μM-N). Faeces extract inartificial seawater was sufficient to sustain E. paludosa growth (up to 6.8 cell divisions in 9 days). A significant growth rate increase (+ 20%) and higher biomasses were observed when faeces extract was added to inorganic media enriched with 50 μM-NO3. Bird droppings had a significant effect in E. paludosa final biochemical composition with the addition of faeces extract to a culture medium containing 50 μM-NO3 increasing E. paludosa protein content and decreasing carbohydrate content. Pigment content per cell increased with the addition of bird dropping extract but ratios of light-harvesting and photo-protective pigments to chlorophyll a were unaffected. E. paludosa grown with faeces extract showed high cellular nitrogen and carbon contents, close to those obtained when cells were grown in F/2 medium. This study showed that shorebird droppings, through the addition of dissolved material, can significantly affect microphytobenthic diatom growth and biochemical composition.

International audience; The strong biological production of estuarine intertidal flats is mainly supported by benthic diatoms in temperate areas. Their photosynthetic productivity is largely driven by changes in light intensity and temperature at the surface of sediment flats during emersion. The impact of an increase in salinity of the upper-layer sediment pore-water during emersion, which is often coupled with high light (HL), has been less studied. Furthermore, benthic diatoms show several growth forms which inhabit specific sediment types where the pore-water salinity can differentially vary due to the degree of cohesion of sediment grains. So far, no study explored if the main growth forms of benthic diatoms (i.e. epipelon, epipsammon and tychoplankton) show different photophysiological response to a combine high salinity-HL stress. Based on field monitoring, we compared the photophysiology (photosynthetic efficiency and photoprotection) of three representatives of the main growth forms during a short high salinity coupled with a moderate HL stress and stable optimal temperature, i.e. experimental conditions reproducing Spring environmental conditions in intertidal flats by the Atlantic French coast. Our results show that all growth forms reacted to HL exposure alone, as expected. While the epipelon representative was relatively insensitive to high salinity alone and combined with HL, the tychoplankton representative was highly sensitive to both, and the epipsammon representative was sensitive mainly to the stress combination. These specific responses fitted well with i) their natural habitat (i.e. more or less cohesive sediment) for which light climate and changes in salinity are different, ii) their growth form (i.e. motile, immotile or amphibious) which determines their probability to be confronted to a combined high salinity-HL stress. Hence, the negative effect of high salinity on photosynthetic efficiency of benthic diatoms appears to be mostly restricted to epipsammon and tychoplankton, and in field conditions, its effect probably remains negligible compared to HL stress.

The aim of this study was to contribute to a better understanding of turbid coastal ecosystems, focusing on spatio-temporal changes at the mesoscale in microphytobenthos structures in a macrotidal bay (Bourgneuf, France). This was based on pigment analysis by high-pressure liquid chromatography (HPLC), reinforced by microscopy observations, in relation to sediment characteris- tics. Microphytobenthos assemblages of this mudflat were 97% dominated by diatoms, confirmed by HPLC analysis, which showed biomarker pigments of this group: chlorophyll c, fucoxanthin, dia- dinoxanthin and diatoxanthin. The main assemblage is constituted of epipsammic species, like Plagiogrammopsis, Achnanthes and Cocconeis, characterising mixed sediments. However, this could be replaced by epipelic species, like Plagiotropis, Pleurosigma and Staurophora, during summer due to station silting. Microphytobenthic photosynthetic active biomass in the first 2 mm varied spatially and temporally from 12.2 to 186.9 mg chl a m -2 . The predominance of a biomass spatial variability reflected spatial changes in sediment grain size and water content. Biomass in the first 2 mm repre- sented from 28 to 68% of the total amount in the first 10 mm. This indicated an accumulation of bio- mass in illuminated layers and a biomass decrease with depth associated with increasing values of the ratio of phaeopigments a and fucoxanthin to chlorophyll a (phaeo a/chl a and fuco/chl a). Both could exceed 1.00 in deeper layers. Temporal changes in microphytobenthos structure at the mesoscale were mainly linked to hydrodynamism. This is stronger in winter and responsible for allochthonous pigment input (lutein and phaeophytin b) and sediment mixing, as grazing activity by macro- and meiofauna, forming chlorophyll a breakdown products, is responsible for an increase in fuco/chl a and phaeo a/chl a values during spring/summer.

Although estuarine microphytobenthos (MPB) is frequently exposed to excessive light and temperature conditions, little is known on their interactive effects on MPB primary productivity. Laboratory and in situ experiments were combined to investigate the short-term joint effects of high light (HL) and high temperature (37 °C versus 27 °C) on the operating efficiency of photoprotective processes [vertical migration versus non-photochemical quenching (NPQ)] exhibited by natural benthic diatom communities from two intertidal flats in France (FR) and Portugal (PT). A clear latitudinal pattern was observed, with PT biofilms being more resistant to HL stress, regardless the effect of temperature, and displaying a lower relative contribution of vertical migration to photoprotection and a stronger NPQ in situ. However, higher temperature leads to comparable effects, with photoinhibition increasing to about three times (i.e. from 3% to 10% and from 8% to 22% in PT and FR sites respectively). By using a number of methodological novelties in MPB research (lipid peroxidation quantification, Lhcx proteins immunodetection), this study brings a physiological basis to the previously reported depression of MPB photosynthetic productivity in summer. They emphasize the joint role of temperature and light in limiting, at least transiently (i.e. during emersion), MPB photosynthetic activity in situ.

Diatoms are usually referred to as golden-brown microalgae, due to the colour of their plastids and to their pigment composition, mainly carotenoids (fucoxanthin, diadinoxanthin, diatoxanthin), which mask chlorophylls a and c. The species Haslea ostrearia Gaillon/Bory (Simonsen) appears unique because of its extraplastidial bluish colour, a consequence of the presence of a water-soluble blue pigment at cell apices, marennine. When released in seawater, marennine can be fixed on gills of oysters and other bivalves, which turn green. This greening phenomenon is economically exploited in Southwestern France, as it gives an added value to oysters. For decades, this singularity ascribed a worldwide distribution to H. ostrearia, first as Vibrio ostrearius, then Navicula ostrearia, last as H. ostrearia, when the genus Haslea was proposed by R. Simonsen (1974). Indeed, this ‘birthmark’ (presence of blue apices) made H. ostrearia easily recognisable without further scrutiny and identification of the microalga as well as its presence easily deduced from the greening of bivalves. Consequently, the widely admitted cosmopolitan character of H. ostrearia has only been questioned recently, following the discovery in 2008, of a new species of blue diatom in the Black Sea, Haslea karadagensis. The biodiversity of blue diatoms suddenly increased with the finding of other blue species in the Mediterranean Sea, the Canary Islands, etc., the taxonomic characterization of which is in progress. This review thus focuses on the unsuspected biodiversity of blue diatoms within the genus Haslea. Methods for species determination (morphometrics, chemotaxonomy, genomics), as well as a new species, are presented and discussed.

L'affinage traditionnel des huitres creuses, pratique dans les claires ostreicoles de la facade Atlantique francaise, est soumis aux fluctuations des facteurs environnementaux. Un procede d'affinage controle en terme de matieres organique et inorganique particulaires (MOP et MIP) et de temperature a ete developpe afin de reduire la variabilite de croissance de la chair des huitres. Cet elevage intensif repose sur la production de la diatomee Skeletonema costatum, distribuee aux huitres avec une concentration moyenne de 4–5 mg de MOP·L–1. Un modele ecophysiologique de l'huitre Crassostrea gigas, simulant les croissances du soma et des reserves–gonades, a ete applique aux conditions d'affinage controle dans le but d'analyser les reponses du bivalve. Son elaboration a necessite un retour a l'experimentation. Deux fonctions d'alimentation ont ete etudiees au laboratoire : la filtration et la production de pseudofeces, intervenant dans la regulation de l'ingestion. Les resultats, pour une temperature de 14 °C, et pour une gamme de MOP et de MIP variant respectivement de 4 a 18 mg·L–1 et de 15 a 55 mg·L–1, montrent que l'ingestion est regulee par la production de pseudofeces, la filtration ne presentant pas de variation significative (moyenne de 2,09 ± 0,11 L·h–1·g–1, avec un taux d'activite de 59%). Cette production de pseudofeces, qui engendre une augmentation de la fraction organique ingeree par le mecanisme de selection pre-ingestive, permet de compenser la variabilite des concentrations en MIP. Des simulations ont permis d’analyser les effets negatifs de ce facteur environnemental sur la croissance du bivalve. Il apparait alors que la ration utilisee en affinage controle (4–5 mg de MOP.L–1) permet une croissance en chair seche malgre des concentrations simulees atteignant 50 mg·L–1 de MIP. Lors de l'elaboration du modele pour les conditions specifiques a l'affinage controle, la gametogenese est apparue comme un processus determinant pour la repartition de l'energie chez le bivalve.

International audience; The objective of the present study was to estimate in vivo pigment composition and to retrieve absorption cross-section values, a∗, of photosynthetic micro-organisms using a non-invasive technique of reflectance spectrometry. To test the methodology, organisms from different taxonomical groups and different pigment composition were used (Spirulina platensis a Cyanophyta, Porphyridium cruentum a Rhodophyta, Dunaliella tertiolecta a Chlorophyta and Entomoneis paludosa a Bacillariophyta) and photoacclimated to two different irradiance levels: 25 μmol photon m−2 s−1 (Low Light, LL) and 500 μmol photon m−2 s−1 (High Light, HL). Second derivative spectra from reflectance were used to identify pigment in vivo absorption bands that were linked to specific pigments detected by high performance liquid chromatography. Whereas some absorption bands such as those induced by Chlorophyll (Chl) a (416, 440, 625 and around 675 nm) were ubiquous, others were taxonomically specific (e.g. 636 nm for Chl c in E. paludosa) and/or photo-physiological dependent (e.g. 489 nm for zeaxanthin in the HL-acclimated S. platensis). The optical absorption cross-section, a∗, was retrieved from reflectance data using a radiative transfer model previously developed for microphytobenthos. Despite the cellular Chl a decrease observed from LL to HL (up to 88% for S. platensis), the a∗ increased, except for P. cruentum. This was attributed to a ‘package effect’ and to a greater absorption by photoprotective carotenoids that did not contribute to the energy transfer to the core Chl a.

International audience; The capacity of microphytobenthos to withstand the variable and extreme conditions of the intertidal environment, prone to cause photoinhibition, has been attributed to particularly efficient photoprotection. However, little is known regarding the capacity of this protection against photoinhibition or the mechanisms responsible for it. The present study quantified the photoprotective capacity and the extent of photoinhibition under excess light, estimated the contribution of vertical migration and the xanthophyll cycle to overall photoprotection, and evaluated the effects of photoacclimation. A new experimental protocol combined (1) chlorophyll fluorescence imaging, for the simultaneous measurement of replicates and experimental treatments, (2) specific inhibitors for vertical migration and for the xanthophyll cycle, to quantify the relative contribution of each process, and (3) recovery kinetics analysis of photosynthetic activity during light stress-recovery experiments, to distinguish rapidly reversible photochemical down-regulation from photoinhibition. The results show a high photoprotective capacity in 2 study periods, May and October, with photoinhibition rates below 20%. A clear change in photoacclimation state was observed, with acclimation to lower irradiances in autumn associated with higher susceptibility to photoinhibition. In May, vertical migration and the xanthophyll cycle provided comparable protection against photoinhibition; in October, the former predominated. The sum of their contributions was ~20% in both months, suggesting that other processes also contribute to photoprotection.